Method for reducing carbon dioxide

ABSTRACT

The method for reducing carbon dioxide of the present disclosure includes a step (a) and a step (b) as follows. A step (a) of preparing an electrochemical cell. The electrochemical cell comprises a working electrode, a counter electrode and a vessel. The vessel stores an electrolytic solution. The working electrode contains at least one carbide selected from the group consisting of zirconium carbide, hafnium carbide, niobium carbide, chromium carbide and tungsten carbide. The electrolytic solution contains carbon dioxide. The working electrode and the counter electrode are in contact with the electrolytic solution. A step (b) of applying a negative voltage and a positive voltage to the working electrode and the counter electrode, respectively, to reduce the carbon dioxide.

This is a continuation of International Application No.PCT/JP2011/002070, with an international filing date of Apr. 7, 2011,which claims the foreign priority of Japanese Patent Application No.2010-099468, filed on Apr. 23, 2010, the entire contents of both ofwhich are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention The present disclosure relates to a method forreducing carbon dioxide.

2. Description of Related Art

A carbon dioxide (CO₂) reduction technique using a catalyst is expectedas a technique for fixing CO₂ and producing useful substances. Thereduction technique is one of the important means for solving theproblem of greenhouse gas-induced global warming believed to besignificant in the future. As the CO₂ reduction techniques using acatalyst, a catalytic hydrogenation method and an electrochemical method(electrolytic reduction method) have been studied so far. In thecatalytic hydrogenation method, CO₂ reacts catalytically with hydrogen(H₂) to be reduced under a high temperature and high pressure gas phasecondition. The catalytic hydrogenation method allows CO₂ to be convertedinto highly useful substances such as methanol (JP 4167775 B and JP1(1989)-313313 A).

In the electrolytic reduction method, the reducing reaction proceedseven at an ordinary temperature and ordinary pressure. The electrolyticreduction method requires no large-scale equipment. Thus, theelectrolytic reduction method is simpler than the catalytichydrogenation method. Accordingly, the electrolytic reduction method isconsidered as an effective CO₂ reduction method. As catalysts capable ofreducing CO₂ by the electrolytic reduction method, metals such as copper(Cu) and silver (Ag), alloy materials of these, and complex materials(molecular catalysts) such as a cobalt (Co) complex, a nickel (Ni)complex and an iron (Fe) complex have been developed so far (Journal ofPhysical Chemistry A Vol. 102 p. 2870 (1998), Journal of AmericanChemical Society Vol. 122 p. 10821 (2000), and Chemistry Letters p. 1695(1985)).

SUMMARY OF THE INVENTION

Generally, CO₂ is a very stable molecule. Thus, the CO₂ reductiontreatment by the catalytic hydrogenation method requires a hightemperature (a heating temperature of 300° C.) and a high pressure (areaction pressure of 50 atmospheres) for a reaction proceeding.Furthermore, the catalytic hydrogenation method uses a flammable gassuch as H₂. For these reasons, the catalytic hydrogenation methodrequires to install large-scale equipment. The catalytic hydrogenationmethod has a problem in that a great deal of energy must be input intothe reduction treatment and in that the energy utilization efficiency isvery low.

Moreover, the metals, the alloy materials, and the molecular materialsused as catalysts in the electrolytic reduction method have a durabilityproblem in that they deteriorate severely with time during the long-timecatalytic reaction. Thus, a catalyst that is capable of reducing CO₂ bythe electrolytic reduction method and has high practicability has notbeen found yet.

One non-limiting and exemplary embodiment provides a method for reducingcarbon dioxide using a highly-durable catalyst that is capable ofreducing CO₂ at an overvoltage equal to or lower than overvoltages forconventional catalysts to produce highly useful substances (such asformic acid (HCOOH), methane (CH₄), ethylene (C₂H₄) and ethane (C₂H₆)).

Additional benefits and advantages of the disclosed embodiments will beapparent from the specification and Figures. The benefits and/oradvantages may be individually provided by the various embodiments andfeatures of the specification and drawings disclosure, and need not allbe provided in order to obtain one or more of the same.

In one general aspect, the techniques disclosed here feature a methodfor reducing carbon dioxide, the method including:

a step (a) of preparing an electrochemical cell, wherein

the electrochemical cell comprises a working electrode, a counterelectrode and a vessel,

the vessel stores an electrolytic solution,

the working electrode contains at least one carbide selected from thegroup consisting of zirconium carbide, hafnium carbide, niobium carbide,chromium carbide and tungsten carbide,

the electrolytic solution contains carbon dioxide,

the working electrode is in contact with the electrolytic solution, and

the counter electrode is in contact with the electrolytic solution; and

a step (b) of applying a negative voltage and a positive voltage to theworking electrode and the counter electrode, respectively, to reduce thecarbon dioxide.

The electrochemical cell is used in the method for reducing carbondioxide of the present disclosure. The electrochemical cell comprisesthe working electrode for reducing carbon dioxide. The working electrodecontains at least one carbide selected from the group consisting ofzirconium carbide, hafnium carbide, niobium carbide, chromium carbideand tungsten carbide. These carbides are capable of reducing carbondioxide at an overvoltage equal to or lower than overvoltages forconventional catalysts for reducing carbon dioxide. Therefore, themethod of the present disclosure makes it possible to produce highlyuseful substances, such as HCOOH, CH₄, C₂H₄ and C₂H₆, at an overvoltageequal to or lower than overvoltages in conventional methods.Furthermore, the high durability of the carbides allows the workingelectrode to achieve high durability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a comparison between an adsorption energy ofcarbon monoxide (CO) on a surface of metal zirconium (Zr) and anadsorption energy of carbon monoxide (CO) on a surface of zirconiumcarbide (ZrC).

FIG. 2 is a structural drawing of an electrochemical cell used formeasurements in the present disclosure.

FIG. 3 is a graph showing the result of a reaction current-electrolyticpotential measurement (C—V measurement) in the case of using zirconiumcarbide (ZrC).

FIG. 4 is a graph showing the result of a gas chromatographic analysisindicating the production of methane (CH₄), ethylene (C₂H₄), and ethane(C₂H₆) in the case of using zirconium carbide (ZrC).

FIG. 5 is a graph showing the result of a gas chromatographic analysisindicating the production of carbon monoxide (CO) and methane (CH₄) inthe case of using zirconium carbide (ZrC).

FIG. 6 is a graph showing the result of a liquid chromatographicanalysis indicating the production of formic acid (HCOOH) in the case ofusing zirconium carbide (ZrC).

FIG. 7A to FIG. 7C are graphs showing the results of reactioncurrent-electrolytic potential measurements (C—V measurements) in thecase of using niobium carbide (Nb₂C), chromium carbide (Cr₃C₂) andtungsten carbide (WC), respectively.

DETAILED DESCRIPTION

Hereinafter, the method for reducing carbon dioxide according to thepresent disclosure will be described with reference to the drawings.

The method for reducing carbon dioxide (CO₂) of the present disclosureis a method for reducing CO₂ electrochemically. In the method of thepresent disclosure, an electrochemical cell is prepared first. Theelectrochemical cell comprises an electrode (working electrode) used toreduce CO₂. The working electrode contains at least one carbide selectedfrom the group consisting of zirconium carbide (ZrC), hafnium carbide(HfC), niobium carbide (Nb₂C), chromium carbide (Cr₂O₃), and tungstencarbide (at least one selected from WC and W₂C). The following is anexample of using zirconium carbide for the working electrode.

Zirconium carbide particles (ZrC particles) obtained by a carbonizationtreatment are dispersed in an organic solvent to prepare a slurrysolution. The ZrC particles have an average particle diameter of aboutseveral micrometers. Then, an appropriate amount of the slurry solutionis applied to a conductive carbon paper (CP) that has carbon fiberswoven therein and is to be used as an electrode substrate. Thus, aworking electrode (catalyst) in which the ZrC particles are supported onthe CP is fabricated. The CP is porous. Therefore, it is difficult tospecify clearly the amount of the supported ZrC particles. However, theamount of ZrC particles supported is about several tens ofmicrograms/cm² to 1 milligram/cm². The electrode substrate is notlimited to the CP as long as it has conductivity. For example, an inertmetal substrate such as a gold (Au) substrate, a glassy carbonsubstrate, and a conductive silicon substrate are commonly used besidesthe CP. Furthermore, the manufacturing method and shape of the ZrCparticles are not limited, either. For example, ZrC having a shape of athin film may be used instead of the ZrC particles mentioned above. Evenin the case of using an electrode structure in which ZrC having a shapeof a thin film is deposited on the surface of the conductive substrateby a method such as sputtering, it is possible to obtain the samecatalytic activity as in the case of using the electrode structure inwhich the ZrC particles are supported on the surface of the conductivesubstrate. Such an electrode production method may cause impurities toenter into the electrode during the production process. However, thecatalytic activity occurs depending on the type of the compound used asa catalyst. Therefore, the impurities which have entered into theelectrode during the production process do not affect the consequence ofthe catalytic activity of the compound.

The configuration of the catalyst for reducing CO₂ containing ZrC isexemplified above. However, as indicated in Examples below, a catalystsample in which hafnium carbide (HfC) particles are supported instead ofthe zirconium carbide particles, a catalyst sample in which niobiumcarbide (Nb₂C) particles are supported instead of the zirconium carbideparticles, a catalyst sample in which chromium carbide (Cr₃C₂) particlesare supported instead of the zirconium carbide particles, and a catalystsample in which tungsten carbide (at least one selected from WC and W₂C)particles are supported instead of the zirconium carbide particles areconfirmed to be effective as catalysts for reducing CO₂.

As described above, the electrode substrate, the shape of the carbidesupported on the substrate, etc. are diverse. However, in the actualreduction treatment of carbon dioxide, an electrolytic reaction in anelectrolytic solution, etc. or an electrolytic reaction utilizing a gasdiffusion electrode is carried out. Therefore, the supporting anddeposition methods are adjusted to be suitable for the carbide so thatthe carbide can be stably supported or deposited on the substrate.

Next will be described the result of analytic evaluation on substancesproduced when CO₂ is subject to the electrochemical treatment using theworking electrode containing the ZrC particles. The substances producedby the CO₂reduction using the working electrode include a gaseouscomponent and a liquid component. In the present embodiment, gaschromatograph is used for analyzing the gas components and liquidchromatograph is used for analyzing the liquid components. As a result,it can be confirmed that CO₂ is reduced to produce CO, HCOOH, CH₄, C₂H₄and C₂H₆. The theoretical background of finding these is as follows.

FIG. 1 shows adsorption energy (E_(a)) of CO on a surface of metalzirconium (Zr) and a surface of zirconium carbide (ZrC) estimated fromsimulations (electronic state calculations) based on density functionaltheory. Generally, in order to cause effectively a catalytic reaction ona surface of a solid matter, it is desirable for the solid matter tohave an appropriate magnitude of E_(a) value. For example, anexcessively large E_(a) value strengthens the absorption of molecules onthe surface of the solid matter, thereby stabilizing the molecules onthe surface of the solid matter. This makes it difficult for a reactionto occur, reducing the possibility of the catalytic reaction occurring.In contrast, an excessively small E_(a) value lowers the probability ofthe molecules being present on the surface of the solid matter. Thisalso reduces the possibility of the catalytic reaction occurring, whichis not desirable. It is known, for example, that metal copper (Cu)causes a reducing reaction of CO₂relatively effectively. It is reportedthat the E_(a) value of CO on a surface of Cu is about −0.62 eV (B.Hammer et al., Physical Review Letter Vol. 76 p. 2141 (1996)).

From this viewpoint, a comparison is made between the metal Zr and thezirconium carbide (ZrC). As shown in FIG. 1, the E_(a) value of CO onthe metal Zr, which is not a compound, is as large as −3.12 eV. Thus, inthe case of using the metal Zr as the working electrode, CO is absorbedstrongly on the surface of the metal Zr, and it is presumed accordinglythat a catalytic reaction hardly proceeds. In contrast, when Zr iscarbonized as in the present disclosure, the E_(a) value of CO islowered to about −0.6 eV. Moreover, a similar calculation confirms thatan adsorption structure is obtained on the surface of ZrC at arelatively small energy also in the case of CO₂ adsorption. Thus, it isconceived that neither the adsorption of CO on the surface of ZrC northe adsorption of CO₂ on the surface of ZrC are too strong and acatalytic reaction occurs very easily.

In a common electrolytic reduction process of CO₂, CO₂ present near asurface of an electrode is reduced by a reaction between electronsinjected from the electrode and protons in a solution. As a result,HCOOH is produced. Moreover, part of CO₂ is reduced to weakly-adsorbedCO by the reaction between the electrons injected from the electrode andthe protons, and the weakly-adsorbed CO further is subject to thereaction between the electrons injected and the protons. As a result,hydrocarbon, such as CH₄, conceivably is produced (Y. Hori et al.,Journal of Chemical Society, Faraday Transaction 1 Vol. 85 p. 2309(1989)).

In view of this, it is conceived that the above-mentioned reactionsproceed also with CO₂ adsorbed on ZrC. As a result, HCOOH, CH₄, C₂H₄ andC₂H₆ conceivably are produced.

On the other hand, the same calculation was made with respect to asurface of Cu. As a result, the adsorption energy of CO₂ on the surfaceof Cu was almost 0. That is, a stable CO₂ adsorption structure is hardlyobtained on the surface of Cu. It is known that in a common reducingreaction process of CO₂, a high overvoltage is needed in a process inwhich one electron moves to a CO₂ molecule and then the CO₂ molecule isadsorbed on a surface of a catalyst. Thus, in case of a catalystcontaining Cu on which CO₂ is not adsorbed stably, a high overvoltage isneeded in the process in which CO₂ is adsorbed on the surface of thecatalyst. In contrast, in the case of metal carbides (ZrC, HfC, Nb₂C,Cr₃C₂, WC and W₂C) used in the method for reducing CO₂ of the presentdisclosure, CO₂ can be adsorbed on the solid surfaces of the metalcarbides at a small adsorption energy as described above. This indicatesthat the above-mentioned carbides are capable of lowering theovervoltage for reducing CO₂.

By exemplifying Zr as a metal element, the principle of the catalyticreaction in reducing CO₂ is explained above. Presumably, the moleculeadsorption process and the catalytic reaction process described aboveare the same for the other metal carbides selected as the catalysts forreducing CO₂ in the present disclosure.

The above-mentioned carbides used as catalysts in reducing CO₂ allowsCO₂to be reduced with an external energy from DC power supply atordinary temperature. Moreover, the method for reducing CO₂ of thepresent disclosure can be applied to methods using a solar cell as anexternal power supply. The catalysts for reducing CO₂ can be applied, bycombination with a photocatalyst, to catalysts that can be used withsolar energy.

The method for reducing CO₂ using the carbides is very simple because itcan be carried out by blowing CO₂ gas into an electrolytic solution orby forming a three-phase boundary with a gas diffusion electrode. Thus,it can be said that the method for reducing CO₂ using the carbides is avery promising technique as an energy-saving measure for CO₂ in placeswhere large-scale equipment cannot be installed in houses andcommunities.

Next, an example of the electrochemical cell used in the method forreducing CO₂ of the present disclosure will be described. Anelectrochemical cell having the same configuration as that of a cell(see FIG. 2) used in Examples below will be exemplified. That is, asshown in FIG. 2, the electrochemical cell of the present embodimentcomprises a working electrode 21, a counter electrode 23 and a vessel28. This vessel 28 stores an electrolytic solution 27. The workingelectrode 21 and the counter electrode 23 are electrically connected toeach other and in contact with the electrolytic solution 27. Theelectrolytic solution 27 contains CO₂. The vessel 28 comprises a solidelectrolyte membrane (for example, cation exchange membrane) 25. Thesolid electrolyte membrane 25 is disposed between the working electrode21 and the counter electrode 23. The solid electrolyte membrane 25separates the vessel 28 into a region of the working electrode 21 and aregion of the counter electrode 23. The electrochemical cell comprisesfurther a gas introduction tube 26 that functions as a gas inlet. Oneend of the gas introduction tube 26 is disposed in the electrolyticsolution 27. In the case of reducing CO₂using this electrochemical cell,performed is the step of applying a negative voltage and a positivevoltage to the working electrode 21 and the counter electrode 23,respectively. In this step, CO₂ is supplied to the electrolytic solution27 through the gas introduction tube 26, for example. The workingelectrode 21 contains at least one selected from the group consisting ofzirconium carbide (ZrC), hafnium carbide (HfC), niobium carbide (Nb₂C),chromium carbide (Cr₂O₃), and tungsten carbide (at least one selectedfrom WC and W₂C). In FIG. 2, the working electrode 21 and the counterelectrode 23 are completely immersed in the electrolytic solution 27.However, the placement of the working electrode 21 and the counterelectrode 23 are not limited to this. The working electrode 21 and thecounter electrode 23 have only to be placed in contact with theelectrolytic solution 27. The electrochemical cell shown in FIG. 2 is athree-electrode cell provided further with a reference electrode 22 forthe measurements in Examples. However, the reference electrode 22 is notnecessary to be provided, because it is not essential to measure thepotential when the electrochemical cell is used for reducing CO₂. Anexample of the material for the counter electrode 23 is metal such asplatinum and nickel, and metal oxide such as Cr₂O₃. By selecting amaterial that has a low overvoltage in an oxygen evolution reaction thatoccurs on the counter electrode 23, it is possible to reduce carbondioxide at a lower applied voltage. The method for reducing CO₂ of thepresent disclosure can be carried out using the cell shown in FIG. 2. Inthis method, an electrochemical cell as shown in FIG. 2 is preparedfirst. Subsequently, a negative voltage and a positive voltage areapplied to the working electrode 21 and the counter electrode 23,respectively. For example, the absolute value of a potential differenceis 2.0 V or more. Through these steps, CO₂ contained in the electrolyticsolution 27 is reduced and thereby highly useful substances can beproduced.

From the disclosure above, the following exemplary embodiments furtherare achieved.

An electrode used to reduce carbon dioxide, the electrode containing atleast one carbide selected from the group consisting of zirconiumcarbide, hafnium carbide, niobium carbide, chromium carbide and tungstencarbide.

A catalyst for reducing carbon dioxide, the catalyst containing at leastone carbide selected from the group consisting of zirconium carbide,hafnium carbide, niobium carbide, chromium carbide and tungsten carbide.

EXAMPLES

In the following examples, the catalyst for reducing CO₂ of the presentdisclosure will be described in further detail.

Example 1

A conductive carbon paper (CP) with a thickness of 0.3 mm was preparedas an electrode substrate. Zirconium carbide particles having an averageparticle diameter of 1 μm (ZrC particles with a purity of 99.9%) weresupported on the CP at a distribution density of about 1×10⁷particles/cm². Thus, the catalyst of the present example was produced.An electrochemical reducing reaction of CO₂ was carried out using thiscatalyst. FIG. 2 shows a schematic view illustrating the structure of anelectrochemical cell used for the measurements in this example. Theelectrochemical cell was a three-electrode cell provided with theworking electrode 21, the reference electrode 22 and the counterelectrode 23. In this cell, the catalyst produced according to thepresent example was used in the working electrode 21. A silver/silverchloride electrode (Ag/AgCl electrode) was used as the referenceelectrode 22. A platinum electrode (Pt electrode) was used as thecounter electrode 23. The electric potential applied to the threeelectrodes was swept by using potensiostat 24, and the reducing reactionof CO₂ was performed and evaluated. As the electrolytic solution 27, 0.1M (0.1 mol/L) potassium bicarbonate aqueous solution (KHCO₃ aqueoussolution) was used. The working electrode 21 and the counter electrode23 were partitioned off with the solid electrolyte membrane 25 toprevent gas components produced by the catalytic activity from mixingwith each other. CO₂ gas was bubbled into the electrolytic solution 27through the gas introduction tube 26 disposed in the cell so as to beintroduced into the electrolytic solution 27.

The measurement was made as follows.

(1) First, nitrogen (N₂) gas was flowed into the electrolytic solution27 at a flow rate of 200 ml/min for 30 minutes. In the state in whichCO₂ was excluded from the solution, the electric potential was swept anda curve of reaction electric current-electrolytic potential (C—V curve)was measured.

(2) Next, the tube was switched from nitrogen gas to CO₂ gas. CO₂ gasalso was flowed similarly into the electrolytic solution 27 at a flowrate of 200 ml/min for 30 minutes. In the state in which theelectrolytic solution 27 was saturated with CO₂, the electric potentialwas swept and the C—V curve under the presence of CO₂was measured.

The difference between the C—V curve obtained in the state (1) (thestate in which CO₂ was excluded from the electrolytic solution 27) andthe C—V curve obtained in the state (2) (the state in which theelectrolytic solution 27 was saturated with CO₂) was calculated. Basedon this difference, a reaction current (hereinafter referred to as areducing current) produced by the reduction of CO₂ was evaluated. FIG. 3shows the results thereof. In this figure, when the current value(vertical axis) is negative, it indicates that the reducing reaction ofCO₂ has occurred. As shown in FIG. 3, the experiment in the presentexample shows that the reaction current fell from zero to a negativevalue where the potential E with respect to that of the silver/silverchloride electrode (Ag/AgCl electrode) was about −0.9 V. That is, in thecase of the catalyst containing ZrC particles, a reducing current of CO₂was observed when the applied voltage was about −0.9 V with respect tothat of the silver/silver chloride electrode (Ag/AgCl electrode). Thismeans that the reduction starts when the applied voltage is about −0.7 Vwith respect to a standard hydrogen electrode. On the other hand, theCO₂ reduction experiment was performed on a catalyst containing singleCu instead of ZrC by using this measurement system. As a result, anapplied voltage higher than −1.1 V was necessary to cause the reducingreaction of CO₂. This result indicates that ZrC is effective in loweringthe overvoltage for reducing CO₂.

Subsequently, the products of the reducing reaction of CO₂ in the caseof using the catalyst containing ZrC particles were analyzed. Gascomponents were analyzed using a gas chromatograph equipped with ahydrogen flame ionization detector (FID). Liquid components wereanalyzed using a UV detection type liquid chromatograph.

FIG. 4 shows the measurement result of methane (CH₄), ethylene (C₂H₄)and ethane (C₂H₆) detected with the FID gas chromatograph. This FID gaschromatograph was equipped with a Porapak Q separation column. The FIDgas chromatograph was programmed so as to control the valve according toa predetermined time sequence, so that CH₄, C₂H₄ and C₂H₆ were detectedafter the elapse of about 1.5 minutes, 4.5 minutes and 6.5 minutes,respectively, from the start of the measurement. As a result, voltagepeaks were observed at corresponding time domains as shown in FIG. 4.This confirmed the production of CH₄, C₂H₄ and C₂H₆. FIG. 5 shows themeasurement result of carbon monoxide (CO), etc. detected with the FIDgas chromatograph. This FID gas chromatograph was equipped with aPorapak N separation column. In this case also, as with the above case,the FID gas chromatograph was programmed so as to control the valveaccording to a predetermined time sequence, so that CO and CH₄ weredetected after the elapse of about 3.2 minutes and 7.2 minutes,respectively, from the start of the measurement. As a result, voltagepeaks were observed at corresponding time domains as shown in FIG. 5.This confirmed the production of CO and CH₄.

FIG. 6 shows the measurement result of formic acid (HCOOH) detected withthe high performance liquid chromatograph. This liquid chromatograph wasequipped with a TSK-GEL SCX (H⁺) column. The liquid chromatograph wasset so that the peak of HCOOH appeared after the elapse of about 11.5minutes from the start of the measurement. As a result, a voltage peakwas observed at this time domain as shown in FIG. 6. This confirmed thatHCOOH also was produced by electrolytic reduction of CO₂ using ZrC.

As described above, the production of CO, CH₄ and HCOOH, and traceamounts of C₂H₄ and C₂H₆ was finally confirmed based on the analysisresults of the products of the catalytic reaction.

Example 2

The same experiment as in Example 1 was conducted also in each of thecases where hafnium carbide (HfC) was used as the catalyst for reducingCO₂, niobium carbide (Nb₂C) was used as the catalyst for reducing CO₂,chromium carbide (Cr₃C₂) was used as the catalyst for reducing CO₂, andtungsten carbide (WC and W₂C) was used as the catalyst for reducing CO₂.As a result, in each case, a reducing current of CO₂ was observed andthe production of CO, CH₄, C₂H₄, C₂H₆, HCOOH, etc. was confirmed, whichis the same as the results obtained when zirconium carbide (ZrC) wasused. Moreover, in the cases where Nb₂C particles, Cr₃C₂ particles andWC particles were used, a reducing current of CO₂ was observed at anovervoltage lower than that for Cu, similarly in the case of using ZrCparticles. FIG. 7A shows the reducing current of CO₂ in the case ofusing the catalyst containing Nb₂C particles. FIG. 7B shows the reducingcurrent of CO₂ in the case of using the catalyst containing Cr₃C₂particles. FIG. 7C shows the reducing current of CO₂ in the case ofusing the catalyst containing WC particles. In the case of using thecatalyst containing Nb₂C particles, the reducing current of CO₂startedto be observed from about −0.85 V with respect to the potential of theAg/AgCl electrode. In the case of using the catalyst containing Cr₃C₂particles, the reducing current of CO₂ started to be observed from about−0.85 V with respect to the potential of the Ag/AgCl electrode. In thecase of using the catalyst containing WC particles, the reducing currentof CO₂ started to be observed from about −0.8 V with respect to thepotential of the Ag/AgCl electrode.

Comparative Example 1

The reducing current of CO₂ was measured using an electrode composedonly of the CP used as the electrode substrate in Example 1. The CO₂reducing current was measured by the same method as in Example 1. As aresult, no CO₂reducing current was observed. That is, the electrodecomposed only of the CP was inactive in the CO₂ reduction. Only oneproduct of the electrolytic reaction was hydrogen (H₂).

Comparative Example 2

As metal carbides other than the metal carbides selected in the presentdisclosure, titanium (Ti) carbide particles and molybdenum (Mo) carbideparticles were produced. These carbide particles were supported on theCP used as the electrode substrate in Example 1 and used as thecatalysts. The reducing current of CO₂ was measured using thesecatalysts. As a result, they exhibited the same properties as those ofthe CP used as the electrode substrate. That is, in the case of usingthe catalysts of Comparative Example 2, only H₂ was produced andproducts such as hydrocarbon and HCOOH were not obtained.

From the results above, it was confirmed that carbides of elementsselected from Zr, Hf, Nb, Cr and W, which are highly durable compounds,were capable of reducing CO₂ electrolytically at an overvoltage lowerthan overvoltages for conventional catalysts. Moreover, it was shownthat the use of these carbides as catalysts for reducing CO₂ made itpossible to obtain CO, CH₄, C₂H₄, C₂H₆, HCOOH, etc. as products. Thesecarbides made it possible to reduce CO₂ electrolytically in anenergy-saving manner, with an external DC power supply at ordinarytemperature.

The catalyst used in the method for reducing CO₂ of the presentdisclosure can be used for more environmentally-friendly configurations.The method for reducing CO₂ of the present disclosure can be applied tomethods using a solar cell as an external power supply. The catalyst forreducing CO₂ can be applied, by combination with a photocatalyst, tocatalysts which can be used with solar energy.

INDUSTRIAL APPLICABILITY

The present disclosure demonstrates that carbides of elements selectedfrom Zr, Hf, Nb, Cr and W, which are highly durable compounds, arecapable of reducing CO₂ electrolytically at an overvoltage lower thanovervoltages for conventional catalysts for reducing CO₂. These carbidesmake it possible to produce CH₄, C₂H₄, C₂H₆, HCOOH, etc. from CO₂ withless energy. That is, the method for reducing CO₂ of the presentdisclosure can provide these useful substances from CO₂ at lower cost.Moreover, the CO₂ reduction treatment technique (the method for reducingCO₂ and the electrochemical cell used in the method) using thesecarbides is also effective as a technique for reducing the amount of CO₂against global warming. The CO₂ reduction treatment technique isexpected to be useful as a more environmentally-friendly resourcerecycling method for the future if they are combined with photocatalytictechnology and solar power generation technology.

1. A method for reducing carbon dioxide, the method comprising: a step(a) of preparing an electrochemical cell, wherein the electrochemicalcell comprises a working electrode, a counter electrode and a vessel,the vessel stores an electrolytic solution, the working electrodecontains, as a catalyst, only at least one carbide, as a catalyst,selected from the group consisting of zirconium carbide, hafniumcarbide, niobium carbide, chromium carbide and tungsten carbide, theelectrolytic solution contains carbon dioxide, the working electrode isin contact with the electrolytic solution, and the counter electrode isin contact with the electrolytic solution; and a step (b) of applying anegative voltage and a positive voltage to the working electrode and thecounter electrode, respectively, to reduce the carbon dioxide.
 2. Themethod according to claim 1, wherein in the step (b), at least onecompound selected from the group consisting of methane, ethylene, ethaneand formic acid is produced.
 3. The method according to claim 1, whereinthe vessel comprises a solid electrolyte membrane, and the solidelectrolyte membrane is interposed between the working electrode and thecounter electrode.
 4. The method according to claim 1, wherein theelectrochemical cell comprises a tube, one end of the tube is disposedin the electrolytic solution, and in the step (b), the carbon dioxide issupplied to the electrolytic solution through the tube.